1. Field of the Invention
This invention relates generally to bandstop filters and more particularly to solid state microwave bandstop filters which utilize both surface and bulk acoustic waves in a piezoelectric medium.
2. Description of the Prior Art
Surface acoustic wave (SAW) devices are often employed as filters or resonators in high frequency applications.
The advantages of using SAW devices over other frequency control methods such as LC circuits, coaxial delay lines, or metal cavity resonators are high Q, low series resistance, small size and good frequency-temperature stability. SAW resonators also possess advantages over bulk acoustic wave (BAW) resonators because the latter must be cut very thin to achieve high frequencies and are consequently quite fragile.
Typically, a SAW device contains a substrate of piezoelectric material such as quartz, lithium niobate, zinc oxide, or cadmium sulfide. Input and output transducers are formed upon the substrate. The transducers convert input electrical signals to surface acoustic waves (SAWs) propagating upon the surface of the substrate and then reconvert the acoustic energy to an electric output signal. The input and output transducers are frequently configured as interdigital electrode fingers which extend from pairs of transducer pads. Interdigital transducers may be formed by depositing a thin film of electrically conductive material upon a piezoelectric substrate.
Alternating electrical potential coupled to the input interdigital transducers induces mechanical stresses in the piezoelectric substrate. The resulting strains propagate away from the input transducer along the surface of the substrate in the form of surface acoustic waves. These propagating surface waves arrive at the output interdigital transducer where they are reconverted to electrical signals.
SAW devices are often used as filters in a variety of applications. Compared to LC filters, for example, SAW devices can provide a narrower passband and the SAW package occupies much less physical space than an LC filter, along with a superior frequency-temperature behavior. Nevertheless, there are applications for which a typical SAW device passband is too broad or the SAW passband includes certain undesirable frequencies. The present invention meets the need for SAW devices with precisely tailored bandstop or band-notch characteristics.
An article pertinent to the understanding of the present invention which discusses the propagation of both surface acoustic waves (also referred to as Rayleigh waves), and bulk acoustic waves is: P. V. H. Sabine, "Rayleigh-wave propagation on a periodically roughened surface," Electronics Letters, Vol. 6, No. 6, Mar. 19, 1970, pp. 149-151. The Sabine article discusses a phenomenon peculiar to surface wave propagation upon a medium with a non-flat surface. The article presents results which show that surface acoustic waves suffer sharp attenuation when travelling over a medium surface which is corrugated in a sinusoidal shape. The attenuation is caused by the sinusoidal corrugations. The corrugations cause a scattering of some of the surface acoustic wave energy into bulk vibration in longitudinal and shear modes. The bulk vibratory modes withdraw energy from the surface, resulting in attenuation of the surface wave.
Sinusoidal corrugations with a fixed periodicity selectively scatter only certain wavelength components of an incident surface acoustic wave spectrum. Other wavelength components of the incident spectrum are not effectively scattered and they traverse the corrugated surface with minimal attenuation. Thus, a series of sinusoidal corrugations cut into the surface of a medium may be used to filter selected surface wave spectral components from an incident SAW spectrum. Undesired spectral components (wavelengths) will be scattered into bulk vibration and will effectively disappear from the surface wave spectrum. The balance of the surface wave spectrum will traverse the corrugations relatively undiminished.
The relationship between SAW attenuation and sinusoidal corrugation wavelength depends upon the Poisson's ratio(s) of the medium. Poisson's ratio is a physical constant which characterizes the behavior of a solid under stress. When a typical isotropic body is stretched in one direction, the body contracts at right angles (i.e. laterally) to the stretch. Poisson's ratio, .sigma., is the ratio of lateral percentage contraction to longitudinal percentage extension. Poisson's ratio can also be expressed as a ratio of material elastic constants or compliances. Values of Poisson's ratio may range from 0 to 0.50. An isotropic body has a single value for Poisson's ratio. However, an anisotropic body, such as quartz or other piezoelectrics commonly employed as substrates for SAW devices, may be characterized by several Poisson's ratios. The Sabine reference presents data appropriate to isotropic media, but the applicability of the results to anisotropic media is apparent to those skilled in the art, and will be illustrated later. For simplicity, the scattering phenomenon will be discussed first in terms of an isotropic medium.
In an isotropic body with Poisson's ratio, .sigma.=0.1, if a surface acoustic wave of wavelength .lambda..sub.R (Rayleigh wavelength) is incident upon sinusoidal corrugations of wavelength .lambda., strong scattering of the incident surface acoustic wave into bulk vibration will occur when 0.5=.lambda..sub.R /.lambda. or when 1.7=.lambda..sub.R /.lambda.. (Actually, Sabine presents scattering data in terms of the parameter .lambda..sub.s /.lambda. where .lambda..sub.s is the medium shear wavelength and .lambda. is the surface corrugation wavelength. However, it is well known to those skilled in the art that the Rayleigh velocity is constrained to be only slightly less than the bulk shear velocity for isotropic media and the Rayleigh velocity is only 2% or 3% less than the slower bulk shear velocity for anisotropic media. Consequently .lambda..sub.R .congruent..lambda..sub.s. For convenience, the invention will be discussed in terms of Rayleigh wavelengths, .lambda..sub.R. Sabine's results are merely normalized in terms of .lambda..sub.s for mathematical convenience.) The amplitude of the surface acoustic wave spectral component of the wavelength .lambda..sub.R after scattering is substantially diminished. The precise amount of attenuation depends upon the number of sinusoidal corrugations encountered by the incident surface acoustic wave. Greater attenuation is produced by a large number of corrugations than by a small number. For Poisson's ratio, .sigma.=0.1, in an isotropic medium, for example, the condition 0.5=.lambda..sub.R /.lambda. yields an attenuation as great as 10 dB per corrugation wavelength. Thus, surface waves of wavelength .lambda..sub.R =0.5.lambda., propagating in such a medium would be attenuated by approximately 100 dB after traversing 10 corrugations.
The aforementioned attenuation data are somewhat dependent upon the Poisson's ratio for the medium, as already mentioned. The Sabine reference illustrates how the SAW attenuation peaks become lower and broader as Poisson's ratio increases from .sigma.=0.1 to .sigma.=0.4. For values of Poisson's ratio .sigma.=0.4, for example, the attenutation per corrugation is between 0.1 dB and 1.0 dB, but the strong-scattering ranges for the ratios .lambda..sub.R /.lambda. are much greater.